To mix or not to mix: Integrated (left) and segregated (right) assemblies were obtained upon treating functionalized γ-Fe(2) O(3) nanoparticles (NPs) with AuNPs. Their binary nature is controlled by the capping layer of the γ-Fe(2) O(3) NPs and the AuNP initial aggregation state. The segregated assembly formation is induced by AuNP aggregates which act as nucleation sites for growth of the γ-Fe(2) O(3) NP domains.